The present invention relates generally to the field of radar mapping and specifically to synthetic aperture radar mapping techniques for generating high resolution maps of surface targets and terrain.
Synthetic Aperture Radar (SAR) is an airborne or spaceborne radar mapping technique for generating high-resolution maps of surface target areas and terrain. The first experimental demonstration of SAR mapping occurred in 1953 when a strip map of a section of Key West, Fla., was generated by frequency analysis of data collected at 3 cm wavelengths from a C-46 aircraft by a group from the University of Illinois. Synthetic aperture radar is used to obtain fine resolution in both slant-range and cross-range. Cross-range resolution refers to resolution transverse to the radar line of sight. The term slant range refers to line-of-sight range to distinguish from cross-range. Resolution in the slant-range to the radar is often obtained by coding the transmitted pulse, typically by FM chirp coding. Cross-range resolution is obtained by coherently integrating echo energy reflected from the ground as the aircraft or spacecraft carrying the radar travels above and alongside the illuminated area to be mapped.
The term synthetic aperture refers to the distance that the radar travels during the time that reflectivity data are collected from a point to be resolved on the earth's surface that remains illuminated by the real antenna beam. Length of the synthetic aperture of a side-looking SAR is the groundtrack distance over which coherent integration occurs. Synthetic length depends on antenna beamwidth and varies with range delay. Synthetic-aperture length in the sidelooking case shrinks for echoes arriving from points closer to the radar and increases for echoes arriving from points farther from the radar. The effect for ideal processing is to produce constant cross-range resolution vs. range. Cross-range resolution is approximately equal to the real aperture's cross-range dimension. FIG. 1 illustrates a sidelooking SAR configuration.
In FIG. 1 aircraft 10 has a sidelooking radar (not shown) that generates a beam 12. The beam 12 has a real antenna beamwidth 14 and there is associated therewith a processed swath width 16 as well as synthetic aperture lengths 18 and 20 at the near and far ranges of the swath width, respectively. In the example illustrated in FIG. 1 the aircraft 10 and associated radar fly along ground track direction 22.
Echo energy collected during illumination of each resolution element of the area to be mapped is made to arrive in phase at the output of the radar processor in order to realize the narrow beamwidth associated with the long synthetically generated aperture. This is achieved by first correcting out all motion of the aircraft that deviated from straight-line motion. At this point the SAR processing is "unfocussed". Then, for "focussed" SAR, the "quadratic" phase error is corrected. Quadratic phase error is produced by straight-line motion of the radar past each point of the mapped area.
It is possible to achieve a second form of SAR, sometimes called "spotlight SAR," illustrated in FIG. 2, in which the radar antenna (not shown) aboard aircraft 10 tracks a particular target area 24 of interest over some azimuth, or cross-range, angle .DELTA..phi.. Here, the cross-range resolution is limited, not by antenna size as for sidelooking SAR, but by target dwell time. Synthetic-aperture length for small .DELTA..phi. can be thought of as the tangential distance the radar travels while moving through the angle .DELTA..phi. to the target.
A third type of SAR is achieved by integrating echo energy as the antenna is scanned in azimuth. This is called "Doppler beam sharpening." Here, for constant azimuth scan rate, relatively long integration time will occur at portions of the scan angle near the direction of platform motion compared to near broadside portions of the scan. This tends to produce constant cross-path resolution and effectively sharpens the real antenna beam.
Some fundamental characteristics of the sidelooking SAR concept can be been explained in terms of equi-range and equi-Doppler lines on the earth's surface to be mapped by a moving radar platform above the earth. As shown in FIG. 3 equi-range lines on the earth's surface are the intersections with the earth's surface of successive concentric spheres centered at the radar. Points on each of these spheres are equidistant from the radar. Equi-Doppler lines 26 on the earth's surface are produced by intersections with the earth's surface of coaxial cones, which are concentric about the flight line 28 of the radar platform 30 as the axis and the radar position as the apex of the cones. Points on each of these cones appear at constant velocity relative to the radar. The zero-velocity cone is a plane perpendicular to the line of flight through the radar's position. The cones for maximum and minimum velocity are straight lines on the flight axis extending ahead or behind the radar, respectively. A flat-earth surface results in a coordinate system made up of the families of the concentric circles and hyperbolas shown in FIG. 3.
At any instant, the radar is able to view that portion of the range/Doppler coordinate system illuminated by the real antenna beam. The distribution of echo power from the illuminated area 32, as a function range delay and Doppler, is the SAR image for the area. Brightness of an image pixel is proportional to the echo power from the corresponding range/Doppler cell on the earth's surface. Mapping resolution is determined by the ability of the radar to measure differential range delay and differential Doppler. Resolution, ideally, is independent of radar range but the image will degrade as thermal or other noise sources begin to determine pixel brightness at low echo signal levels.
A real aperture sidelooking radar is shown in FIG. 4. A real aperture radar maps by resolving in range using relatively short pulses or pulse compression and in azimuth using a long aperture facing off the side of the aircraft.
The echo energy from any element of the area to be mapped must arrive in phase to the radar processor, as in the real aperture radar, in order to realize the narrow beamwidth associated with a long synthetically generated aperture. This is achieved by first correcting out all motion of the aircraft which deviates from straight line motion. Then, the quadratic error may also be corrected. Quadratic error is produced by straight-line motion of the radar past each element which results in a range change to each element of the mapped area. Processes for correcting out this error are well known and are more fully described in HIGH RESOLUTION RADAR, by Donald R. Wehner, published March, 1987, hereby incorporated by reference in its entirety.
Present implementations of all SARs involve some means for pulse compression to obtain the high resolution in slant-range. Pulse compression techniques require wide instantaneous bandwidth throughout the entire radar system which may limit resolution. Large phase steered arrays for space and airborne applications cannot support large bandwidths without costly and lossy linear delay sections to produce true time delay beam steering. Thus, present SAR capabilities and techniques are precluded for use for these systems. Flexible, variable resolution "zoom" capabilities are not now possible with present SAR techniques. Resolution can only be changed in steps, a separate pulse compression waveform for each step being required. Finally, both resolution and dynamic range are limited by digital conversion due to the analog-to-digital conversion speed limitations.